Thin film encapsulation of MEMS devices

ABSTRACT

A method of manufacturing a miniature electromechanical system (MEMS) device includes the steps of forming a moving member on a first substrate such that a first sacrificial layer is disposed between the moving member and the substrate, encapsulating the moving member, including the first sacrificial layer, with a second sacrificial layer, coating the encapsulating second sacrificial layer with a first film formed of a material that establishes an hermetic seal with the substrate, and removing the first and second sacrificial layers.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/401,962, filed Apr. 12, 2006, which is now U.S. Pat. No. 7,638,429,which is a continuation of U.S. application Ser. No. 10/076,559, filedon Feb. 19, 2002, and is now U.S. Pat. No. 7,045,459, the entirecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to micro-electromechanicalsystems (MEMS) and, in particular, to a method of encapsulating one ormore MEMS devices formed on a substrate.

2. Discussion of the Background Art

Switching devices are used in electronic applications to connect anddisconnect electrical signal paths. In radio frequency (RF) andmicrowave applications, it is desirable to form switches usingmicro-electromechanical system (MEMS) technology because such switchesexhibit low insertion loss and high isolation capability compared totransistor switches especially at frequencies in the GHz and above. Atypical MEMS switch, such as described in U.S. Pat. No. 5,619,061 toGoldsmith et al., includes an electrode in the form of a horizontal beamelement with at least one end clamped, hinged or anchored to a post,spacer, via or other type of stationary vertical structure. Anotherelectrode is disposed in opposed relation to the beam element so that,when an appropriate voltage is applied between the two electrodes, thebeam element flexes in the direction of the opposed electrode, therebyclosing the circuit. When the voltage is removed, the natural resiliencyof the beam element returns it to its normally horizontal, open state.

A disadvantage of MEMS devices in general is their sensitivity toenvironmental conditions. More specifically, since MEMS devices operatethrough the mechanical movement of a moving member, the gap between themoving member and the opposed electrode must be clear of any particulatematter that may inhibit movement of the member. The components of a MEMSdevice must also be free of stiction-causing films that can develop whenthe device is exposed to certain environments. Stiction is an adhesiveor electrostatic attraction between electrodes that can have a negativeimpact on the switching speed and response of the device. When thevoltage potential is removed, the electrodes should separateinstantaneously. Any residual, unwanted attraction between electrodeswill increase the time for separation, thereby decreasing switchingspeed. In extreme cases, stiction may bind a MEMS switch in the closedposition, thus rendering it inoperable.

One way to avoid environmental effects is to incorporate a MEMS devicewith other components as part of a system and to enclose the entiresystem in a hermetically sealed package. A disadvantage of this approachis that it dramatically increases the size and cost of the system and,in addition, tends to limit the position of the MEMS device to anexposed surface of the system. The increase in size also necessitateslong vias to connect the MEMS device with radiators and other componentsleading to RF losses and increased noise, particularly at highfrequencies (i.e., at or above 10 GHz).

Another way to avoid environmental effects is to individually encloseone or more MEMS devices in a hermetically sealed package prior toincorporating the devices into a system with other components. Ceramicpackages with metal seals have been proposed for this purpose. A problemwith this approach is that the mating process requires significanthandling of the MEMS devices in an unprotected condition. If a number ofMEMS devices are formed in a batch on a single wafer, it is alsonecessary to saw the wafer with the MEMS devices in an unprotectedcondition thereby increasing the risk of damage to the devices. It isalso difficult to achieve and maintain a hermetic seal because of thevariety of materials used.

Yet another approach is to form a number of MEMS devices on a firstwafer and an equal number of cavities in a second wafer and to usewafer-bonding technology to join the wafers together such that the MEMSdevices are each disposed within a cavity in the opposed wafer. Adisadvantage of this approach is that it requires precise alignment ofthe wafers when bonding and complicates subsequent sawing operations.This approach also leads to a significant increase in the overall sizeof each MEMS device because of the need to use a relatively thick waferfor creation of the cavities.

Therefore, a need exists for an hermetically sealed MEMS device that issmall, inexpensive, easy to produce, and reliable.

SUMMARY OF THE INVENTION

A first aspect of the present invention is generally characterized in amethod of manufacturing a micro-electromechanical device comprising thesteps of forming a moving member on a first substrate such that a firstsacrificial layer is disposed between the moving member and thesubstrate, encapsulating the moving member, including the firstsacrificial layer, with a second sacrificial layer, coating theencapsulating second sacrificial layer with a first film formed of amaterial that establishes an hermetic seal with the substrate, andremoving the first and second sacrificial layers. In a preferredembodiment, an opening is formed in the first film to facilitate removalof the sacrificial layers, and the opening is sealed after thesacrificial layers have been removed. In one embodiment, a conductivelayer is formed on the first film and connected to a circuit to act as acounter electrode.

A second aspect of the present invention is generally characterized in amicro-electromechanical system (MEMS) device comprising a firstsubstrate, a first control circuit formed on the first substrate andincluding a first actuation element, a movable member formed on thefirst substrate in spaced relation to the first actuation element, themovable member being electrically conductive and movable in thedirection of the first actuation element, and a helmet defining ahermetically sealed chamber around the movable member, the helmet beingformed by removing a sacrificial layer between the movable member andthe helmet. In a preferred embodiment, the helmet has tapered side wallsand is formed of a silicon oxynitride film.

A third aspect of the present invention is generally characterized in amethod of fabricating a micro-electromechanical system (MEMS) devicecomprising the steps of forming a control circuit with an actuatingelement on a substrate, defining a movable member above the actuatingelement by applying a first sacrificial layer over the actuatingelement, depositing a conductive material such that the material extendsfrom the circuit to cover the first sacrificial layer, and removingportions of the sacrificial layer around the movable member but notbetween the moving member and the substrate, encapsulating the movingmember on all sides with a second sacrificial layer, coating the secondsacrificial layer with a material that forms an hermetic seal with thesubstrate, and removing the first and second sacrificial layers.

Some of the advantages of the MEMS devices according to the presentinvention are that they are sealed against particulate and vaporcontamination while still in the controlled environment of a clean room,they can be sawed, handled and integrated into electronic systems usingstandard IC techniques without being contaminated or otherwise damaged,they do not need to be positioned on a top surface to be verticallyintegrated, and they can significantly reduce the size and cost ofelectronic systems employing MEMS devices by eliminating the need tohermetically seal the entire system.

The above and other features and advantages of the present inventionwill be further understood from the following description of thepreferred embodiments thereof, taken in conjunction with theaccompanying drawings wherein like numerals are used to denote likeparts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the followingfigures, in which:

FIG. 1 is a plan view and FIGS. 2-9 are cross-sectional viewsillustrating a method of fabricating a micro-electromechanical system(MEMS) device in accordance with a first embodiment of the presentinvention.

FIGS. 10-12 are cross-sectional views illustrating a method offabricating an micro-electromechanical system device in accordance witha second embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A method of manufacturing a MEMS device according to a first embodimentof the present invention is illustrated in FIGS. 1-9. The method can beused to manufacture a variety of MEMS devices but is particularlyadvantageous when used to manufacture a two electrode switch for radiofrequency (RF) applications as shown and described herein.

In the first step, illustrated in FIGS. 1 and 2, a first signal network20 and anchor pads 22 for a movable member are formed on a substrate 24.Bond pads 26 for a control circuit 28 can also be formed on thesubstrate 24 in this step. The first signal network 20 shown in FIG. 1includes a pair of bond pads 30 located at opposite edges of thesubstrate 24 and a stripline 32, which also acts as an actuatingelement, which is described in detail below extending between the bondpads 30 to function as a first electrode. The first signal network 20and pads 22 can be formed on the substrate 24 in any conventional mannerbut are preferably formed using a lift-off procedure wherein aconductive base layer is formed on the substrate and etched through atemplate defined by a layer of photoresist that is later removed. Thesubstrate 24 can be any suitable material including, but not limited to,a semiconductor material, such as silicon or gallium arsenide, aceramic, a glass-reinforced plastic, or a plastic material. Theconductive base layer material can be any conductive material but ispreferably a metal such as copper, platinum, gold, aluminum or alloysthereof.

In the second step, illustrated in FIG. 3, a dielectric layer 34 isdefined over at least a portion of the stripline 32. In the exampleshown, the dielectric layer 34 is formed over the entire stripline 32.The dielectric layer 34 functions as an electrically insulative barrierbetween the stripline 32 and the moving member (not shown) to preventmetal to metal contact that would result in DC shorting as described ingreater detail below. The dielectric layer 34 can be formed of anysuitable dielectric material but is preferably silicon dioxide orsilicon nitride. The dielectric layer 34 is preferably applied usingplasma enhanced chemical vapor deposition (PECVD) but can be applied inany conventional manner. It should be noted that the dielectriclayer-defining step is optional when manufacturing a three electrodedesign (not shown) wherein the control circuit acts through a thirdelectrode that is offset from the stripline as opposed to acting throughthe stripline itself.

In the third step, illustrated in FIG. 4, the control circuit 28 isdefined on the substrate if it has not already been formed in the firststep. In the example shown in FIGS. 1 and 4, the control circuitincludes first and second conductive paths 36 and 38. The firstconductive path 36 extends from a control bond pad 26 to one of theanchor pads 22, and the second conductive path 38 extends from thestripline 32 to the other control pad 26 which goes to ground. Thecontrol circuit 28 is configured to present as an open circuit to RFsignals while carrying a DC voltage potential. To this end, theconductive paths 36 and 38 of the control circuit are preferably made ofa conductive material with high sheet resistance or inductance, such assilicon chrome monoxide or polysilicon, that causes the control circuitto look like an open circuit to RF signals. The conductive paths 36 and38 can be formed in any conventional manner but are preferably formed bya process known as physical vapor deposition (PVD) (i.e., sputtering).

In the fourth step, illustrated in FIG. 5, a first sacrificial layer 40is applied over the product of the third step to function as a mold uponwhich a moving member can be formed. In the example shown, the firstsacrificial layer 40 has a pair of tapered openings 42 exposing theanchor pads. The thickness of the first sacrificial layer 40 defines thegap between the moving member and the signal line. The taper of theopenings 42 defines the geometry of the posts that support the movingmember. The sacrificial layer can be photoresist, Si02, amorphoussilicon, germanium or any other material that can be removed in a laterstep without damaging the other layers. The preferred technique forforming a sacrificial layer 40 with tapered openings 42 is to spinphotoresist onto the product of the third step to obtain a substantiallyuniform layer of photoresist on the substrate, to develop thephotoresist using an appropriate mask that creates openings over theanchor pads, and to bake the developed photoresist so that the walls ofthe openings contract differentially through the thickness of thephotoresist to create the desired taper. In an exemplary embodiment, thedeveloped photoresist is baked at 120-180° C. for up to about 30 minutesdepending upon the type of photoresist. Another alternative is to formthe tapered openings using gray scale photolithography.

In the fifth step, illustrated in FIG. 6, the moving member 44 is formedusing a conductive material such as gold, platinum, copper or aluminum,or a semi-conductor material such as polysilicon or single crystalsilicon. One technique for forming the moving member 44 is by a lift-offprocedure wherein a layer of the conductive material is formed on thefirst sacrificial layer 40 and etched through a template defined by alayer of photoresist that is formed on the sacrificial layer and laterremoved. In the example shown, the procedure results in a moving member44 having a beam element 46 supported at opposite ends by a pair ofposts 48 that extend upwardly from the anchor pads 22 at an angletowards one another.

In the sixth step, illustrated in FIG. 7, a second sacrificial layer 50is defined over the moving member 44. The preferred technique forforming the second sacrificial layer 50 is to apply photoresist over theproduct of the fifth step, to develop the photoresist, and to bake thedeveloped photoresist so that the sacrificial layer contractsdifferentially through the thickness of the photoresist to createtapered sides 52 that reduce the overall size and volume of the device.The resulting layer 50 extends over the top and around the sides of themoving member 44 to serve as a mandrel upon which a helmet can be formedin spaced relation to the moving member.

Referring still to FIG. 7, a helmet or cover 54 is formed by depositinga first encapsulant layer 55 over the second sacrificial layer 50. Thehelmet 54 defines a hermetically sealed chamber containing the movingmember 44. The helmet 54 can be made of any encapsulant material capableof forming an hermetic seal with the substrate 24 and the variouscircuit components but is preferably a low temperature, low stress PECVDfilm such as silicon oxynitride. The preferred deposition temperaturefor this process is less than 150° C. depending upon the photoresistprocessing method. The gases used in the PECVD deposition of siliconoxynitride are preferably silane, nitrous oxide, ammonia and a carrierof nitrogen or argon. The silane, nitrous oxide and ammonia are reactivegases that decompose during the deposition leaving nitrogen gas insidethe helmet 54 at a reduced pressure. The pressure differential betweenthe nitrogen gas inside the helmet 54 and the atmosphere places thehelmet into compression which is a desirable condition for theencapsulant material.

At some point during or after deposition of the encapsulant, one or moreopenings 56 are formed in the first helmet layer 55 as shown in FIG. 7.The openings 56 can be formed during the encapsulation by masking theareas where openings are desired. After encapsulation, openings can beformed by masking the helmet and etching through the helmet materialwith an appropriate etchant, for example, by reactive ion etching usingetchants such as a cholorofluorocarbon or SF₆ gas or etching with ahydrofluoric acid based wet etch solution. The size, shape, number andlocation of the openings 56 are chosen to facilitate introduction of amaterial into the helmet 54 for removal of the sacrificial layers 40 and50 and to allow subsequent sealing of the helmet. For example, a singlerectangular opening about half the height of the moving member can beformed on one side of the helmet as shown in FIG. 7. Preferably, a pairof rectangular openings are formed on opposite sides of the helmet.

In the seventh step, illustrated in FIG. 8, the first and secondsacrificial layers 40 and 50 are removed via the opening 56 in thehelmet 54. The preferred technique for removing the sacrificial layers40 and 50 is to immerse the product of the sixth step in a material thatreacts with the sacrificial layers but not with the helmet material. Forexample, if the sacrificial layers are formed of photoresist, theproduct can be immersed in standard photoresist stripper. If, on theother hand, the sacrificial layers are formed of amorphous silicon, amaterial such as xenon difluoride may be used. Other alternativesinclude placing the product in a vacuum chamber to remove air andintroducing a reactive gas, or immersing the product in a supercriticalfluid, such as liquid CO₂. It should also be noted that use of atransparent helmet material, such as silicon oxynitride, permits visualobservation of the progress of the removal process so that completeremoval of the sacrificial layers can be confirmed.

Once the sacrificial layers have been removed, the elements inside thehelmet 54 can optionally be treated with an anti-stiction coating, suchas a self-assembled monolayer, to make the surfaces of the elementshydrophobic and to increase the life of the device.

In the eighth step, illustrated in FIG. 9, the openings 56 formed in thehelmet 54 are sealed. A preferred technique for sealing the helmetopenings is to deposit a second encapsulant layer 57 over the firstencapsulant layer 55 such that the second encapsulant layer extends overthe openings and forms a seal with the first layer and the substrate 24.The second encapsulant layer 57 can be made of any material capable ofsealing openings in the first encapsulant layer 55 but is preferablymade of the same material as the first encapsulant layer and formed inthe same manner as the first encapsulant layer. Using the same materialfor the first and second encapsulant layers ensures that both layershave the same coefficient of thermal expansion such that interlaminarstresses that could lead to cracking and leaks are minimized.

If bond pads have been formed on the substrate, the next step is toremove the helmet material covering the bond pads as a result of earliersteps so that electrical connections can be made to the resulting MEMSdevice 60.

The resulting MEMS device 60 includes an electrically conductive movingmember 44 supported above a signal line 32 on a substrate 24 within ahermetically sealed chamber defined by a helmet 54. Electricalconnections between the moving member 44, the signal line 32 andexternal circuits (not shown) can be made using the bond pads 30illustrated in FIG. 1. In general, the control circuit 28 is used tocontrol the position of the moving member relative to the signal line 32actuating element by varying the voltage potential between theseelements. The signal line 32 is connected to circuitry that carries asignal affected by the position of the moving member 44. Because theMEMS device 60 exhibits extremely low loss, it is ideal for use in RFapplications as a switch or a tunable capacitor. The device 60 can alsobe used in a phase shifter, a time delay unit, and other devices andsystems.

MEMS devices produced in accordance with the present invention can besignificantly thinner than those produced using standard wafer bondingtechniques. In an exemplary embodiment, the overall height of a MEMSdevice according to the present invention is about 12 μm from thesurface of the substrate to the top of the helmet, with the base metalhaving a thickness of about 0.5 μm, the dielectric layer having athickness of about 0.5 μm, an air gap of about 2 μm between thedielectric and the moving member, the moving member having a thicknessof about 2 μm, an air gap of about 2 μm between the moving member andthe helmet, and the helmet having a thickness of about 5 μm.

Another method of manufacturing a MEMS device according to a secondembodiment of the present invention involves performing the stepsillustrated in FIGS. 10-12 after the steps illustrated in FIGS. 1-7. Theresulting MEMS device 70 includes a counter electrode 72 embedded in thehelmet 54 in addition to an electrode 32 on the substrate 24 such thatthe moving member 44 is disposed between a pair of electrodes.

The first through sixth steps of the second embodiment are performed inthe same manner as described above to provide an intermediate productwith a first encapsulant layer 55 disposed over and around the movingmember 44 and the sacrificial layers 40 and 50.

In the seventh step of the second embodiment, illustrated in FIG. 10, acounter electrode 72 is defined on the first encapsulant layer 55, andan optional second encapsulant layer 57 is defined on the firstencapsulant layer thereby covering the counter electrode. The counterelectrode 72 can be formed of any electrically conductive material inany suitable manner but is preferably formed of the same material as thesignal line 32 using a lift-off technique or by coating the encapsulantlayer 55 with a conductive layer and etching the counter electrode. Thesecond encapsulant layer 57 can be made of any material to cover thecounter electrode 72 but is preferably formed of the same material asthe first encapsulant layer 55 using the same application technique. Thesecond encapsulant layer 57 is desirable to protect the counterelectrode 72 during subsequent process steps but is optional. The thirdencapsulant layer 74 described below can be used to cover the counterelectrode 72 if a second encapsulant layer is not applied.

In the eighth step of the second embodiment, illustrated in FIG. 11, thesacrificial layers 40 and 50 are removed from within the helmet 54 viaan opening 56 in the same manner as described above, for example, byimmersing the MEMS structure in photoresist stripper.

In the ninth step of the second embodiment, illustrated in FIG. 12, athird encapsulant layer 74 is deposited to seal gaps in the first andsecond layers 55 and 57 of encapsulant. The third encapsulant layer 74can be formed of any material capable of sealing the gaps but ispreferably formed of the same material as the first and, secondencapsulant layers 55 and 57 and in the same manner. At this point, thebond pads 26 and 30 are etched open as before.

The resulting MEMS device 70 has an electrically conductive movingmember 44 disposed within a hermetically sealed chamber defined by ahelmet 54 on a substrate 24, the moving member being supported between aprimary electrode defined by a signal line 32 on the substrate and acounter electrode 72 mounted on the helmet. Electrical connectionsbetween the moving member 44, the signal line 32 and external circuits(not shown) can be made using the bond pads 30 illustrated in FIG. 1.One or more additional bond pads can be formed in the same manner forthe counter electrode 72. The signal line 32 and first control circuit28 are connected as described previously. The counter electrode 72 canbe electrically connected to a second control circuit and/or a secondsignal circuit depending upon the desired application. By formingelectrodes on opposite sides of the moving member 44, the moving membercan be made to move up or down from its rest position. Thus, thepresence of a counter electrode 72 on a side of the moving member 44opposite the signal line 32 increases the effective range of movement ofthe moving member and, thus, the frequency range of operation of thedevice 70 without increasing the gap between the moving member and thesubstrate 24 or the actuation voltage. The counter electrode 72 alsoallows the gap between the moving member 44 and signal lines 32 to beprecisely defined in a repeatable manner even when the moving membersuffers from effects such as creep, plastic deformation, and fatigue.The design also provides a dielectric layer (i.e., the first encapsulantlayer 55) to prevent a DC short when the moving member approaches thecounter electrode.

The method according to either embodiment of the invention can beperformed to produce multiple MEMS devices of the above type on a singlesubstrate. The MEMS devices can all be enclosed within a single helmet,or divided between multiple helmets. If the MEMS devices are disposedwithin multiple helmets on a single substrate, it is possible to cut anddice the substrate between helmets so as to separate the devices withoutfear of contamination since the moving members are contained withinindividually sealed helmets that protect internal elements from outsideconditions. MEMS devices fabricated according to the above method can behandled with conventional pick and place machinery without the need formodification such that, for example, the devices can be wire bonded,connected using flip chip technology, or ceramic bonded to a circuit asdesired without fear of contamination. Because the moving member isprotected, the MEMS device need not be formed on the top surface of anassembly, allowing close vertical integration of the MEMS device withother components and devices.

While the invention has been described in detail above, it will beappreciated that various modifications can be made without departingfrom the scope of the invention as defined in the following claims. Forexample, the moving member can be a simply supported double-clamped beamfixed at both ends as shown, a cantilever beam member fixed at one end,a diaphragm (edge-supported) member, or any other type of movablemember. The moving member can be actuated by electrostatic forcesgenerated by creating a DC voltage potential between the moving memberand a control electrode, by piezoelectric, magnetic or thermal elementsmounted on the moving member and powered by a control circuit, or byexternal physical stimuli such as acceleration. The signal and controlnetworks can be defined on the same side of the substrate as the movingmember as shown, or portions of the networks can be defined on anopposite side of the substrate using vias through the substrate toconnect with portions on the same side as the moving member. The helmetcan have any shape including, but not limited to, a tapered shape asshown, a rounded dome shape, or a generally rectangular, box-like shape.The signal network can include a stripline as shown or segmentsseparated by a gap that is closed by movement of the moving member.

1. A micro-electromechanical system (MEMS) device comprising: a firstsubstrate having a top planar surface; a first control circuit formed onthe top surface of said first substrate and including a first actuationelement; a movable member formed on said first substrate in spacedrelation to said first actuation element, said movable member beingelectrically conductive and movable in the direction of said firstactuation element; and a helmet defining a hermetically sealed chamberaround said movable member, said helmet being formed by removing asacrificial layer between said movable member and said helmet.
 2. TheMEMS device of claim 1, and further comprising an inert gas disposedwithin said hermetically sealed chamber.
 3. The MEMS device of claim 1,and further comprising a second control circuit with an actuator elementdisposed within said helmet.
 4. The MEMS device of claim 1, and furthercomprising a plurality of moving members formed on said substrate,wherein said helmet defines a plurality of hermetically sealed chambersaround said movable members.
 5. The MEMS device of claim 1, wherein saidhelmet is formed of a silicon oxynitride film.